The present invention generally relates to processes and systems for removing acidic gases from flue gases produced by power generation, industrial, and other facilities. More particularly, this invention is directed to flue gas desulfurization (FGD) processes and systems that utilize ammonia to capture sulfur dioxide and produce ammonium sulfate, with the further capability of preventing or at least reducing the release of hydrogen sulfide and/or mercaptans into a scrubbed flue gas stream produced by such processes and systems.
Acidic gases, including sulfur dioxide (SO2), hydrogen chloride (HCl) and hydrogen fluoride (HF), are known to be hazardous to the environment, and as a result their emission into the atmosphere is closely regulated by clean air statutes. For the removal of acidic gases from flue gases produced by utility and industrial plants, gas-liquid scrubbers (contactors, absorbers, etc.), are widely employed. Scrubbers generally employ a liquid-containing media that is brought into intimate contact with a flue gas to remove acidic gases by absorption. The process by which acidic gases are removed from flue gases in this manner is generally referred to as wet flue gas desulfurization (wet FGD).
The cleansing action produced by scrubbers is generally derived from the passage of a flue gas through a tower cocurrently or countercurrently to a descending liquid medium. Calcium-based slurries, sodium-based solutions and ammonia-based solutions are typical alkaline scrubbing media used in flue gas scrubbing operations. The cleansed gases are allowed to exit the tower, typically passing through a mist eliminator to atmosphere. The liquid medium and its absorbed gases are collected in a tank, typically at the bottom of the tower, where the absorbed gases are reacted to form byproducts that are useful or at least not harmful to the environment. While scrubbers utilizing calcium-based slurries generally perform satisfactorily, their operation results in the production of large quantities of wastes or gypsum, the latter having only nominal commercial value.
In contrast, ammonia-based scrubbing processes have been used to produce a more valuable ammonium sulfate fertilizer, as taught by U.S. Pat. Nos. 4,690,807 and 5,362,458, each of which are assigned to the assignee of the present invention and incorporated herein by reference. In these processes (also known as ammonium sulfate flue gas desulfurization, or AS FGD), the scrubbing solution is accumulated in a tank where the absorbed sulfur dioxide reacts with dissolved ammonia (NH3) to form ammonium sulfite ((NH4)2SO3) and ammonium bisulfite (NH4HSO3), which are oxidized in the presence of sufficient oxygen to form ammonium sulfate ((NH4)2SO4) and ammonium bisulfate (NH4HSO4), the latter of which reacts with ammonia to form additional ammonium sulfate. A portion of the ammonium sulfate solution and/or ammonium sulfate crystals that form in the solution can then be drawn off to yield the desired byproduct of this reaction.
Ammonia produced from sour water in refinery, tar sands and other similar applications typically contains low concentrations of hydrogen sulfide (H2S) and mercaptans (or thiols, which are compounds that contain the functional group composed of a sulfur atom and a hydrogen atom (—SH)). When the ammonia is used in an ammonium sulfate FGD system to capture sulfur dioxide, the hydrogen sulfide and mercaptans are stripped from the ammonia because of their volatility in ammonium sulfate, and thereafter become entrained with the flue gas. Scrubbed flue gases that exit the FGD system contain the entrained hydrogen sulfide and mercaptans which, in addition to being air pollutants, can contribute an undesirable odor to the FGD emissions.
Current technologies employed to purify ammonia and separate it from hydrogen sulfide and mercaptans are expensive capital-intensive processes, and consume considerable amounts of energy. As such, it would be desirable if an improved process were available that was capable of greatly reducing the amounts of hydrogen sulfide and mercaptans released from AS FGD systems, while also avoiding the disadvantages of prior art processes.